This invention relates to a lithium ion secondary battery, a cathode active material therefor comprised of a lithium cobalt composite oxide and a method for the production of the composite oxide. More particularly, the invention relates to a lithium ion secondary battery which comprises a lithium cobalt composite oxide containing a substitution element as a cathode active material and which has improved cycle characteristics and thermal stability, the lithium cobalt composite oxide as a cathode active material and a method for the production of the composite oxide.
Backed by the recent demands for small sized and high performance electronic appliances, the demand for safe, high voltage, high energy density and light-weight lithium ion secondary batteries having a long life rapidly increases. Investigation is also intensively made to put lithium ion secondary batteries to practical use in the field of large scale electric power storage systems or electric mobiles to cope with world-wide decrease in resources and environmental pollution.
A lithium cobalt composite oxide (LiCoO2, lithium cobaltate) has been so far widely used as a high performance cathode active material of lithium ion secondary batteries since it has a 4 volt high voltage and high energy density. However, when such a lithium ion secondary battery is subjected to repetition of charge and discharge process, the crystal structure of lithium cobalt composite oxide is gradually destroyed to fail to function as a cathode active material and hence the battery has no satisfactory cycle performance.
Furthermore, since the lithium cobalt composite oxide decomposes at a temperature of about 200xc2x0 C., there is a fear that the battery is broken when there arises unusual heat generation on account of, for example, internal short circuit. The battery has a further disadvantage in that the cycle characteristics deteriorates rapidly at high temperatures and hence the battery lacks heat stability.
Under these circumstances, it has been proposed to convert the lithium cobalt composite oxide to such a substituted composite oxide in which a part of cobalt atoms are substituted for an element other than cobalt atom thereby to suppress undesired change of the crystal structure while the battery is subjected to charge-discharge cycles.
The substituted lithium cobalt composite oxide has heretofore been produced by dry-blending and pulverizing powders of raw materials such as lithium carbonate, cobalt carbonate and an oxide of an element to substitute in part the cobalt atoms or a substitution element, firing the resulting mixture, cooling, and then pulverizing the fired product, as described in, for example, Japanese Patent Application Laid-open No. 3-201368, No. 4-319259 or No. 5-283075.
However, according to such a method which employs a solid-state reaction using powders as reactants, it is difficult to obtain a uniform mixture of the powders having a particle size of a micron level. Moreover, since the thermal conductivities of the powders of carbonates or oxides which are usually used as raw materials are very low. Accordingly, for the production of practically usable composite oxides by firing such a mixture of powders of the raw materials, it is necessary to use a relatively large amount of powder of the substitution element and fire the resulting mixture for a long time at a high temperature followed by pulverizing the fired product. In addition, it is still necessary to repeat such firing and pulverizing operations so that the desired solid-state reaction is completed.
On the other hand, when such a mixture of powders containing a large amount of a compound of the substitution element is repeatedly fired at a high temperature, undesired by-products are often formed or the surface properties of the resultant product undesirably change with the result that lithium ion secondary batteries obtained using the resulting composite oxide as a cathode active material have a small cell capacity. Thus, the known composite oxides have failed to provide a high performance battery.
The invention has been accomplished to solve the problems involved in the known substituted lithium cobalt composite oxides for use as a cathode active material. Therefore, it is an object of the invention to provide a substituted lithium cobalt composite oxide which has not only uniform composition and improved heat stability but also excellent cycle characteristics and enables the manufacture of high capacity secondary batteries. It is a further object of the invention to provide a lithium ion secondary battery in which the substituted lithium cobalt complex oxide is incorporated as a cathode active material.
The invention provides a substituted lithium cobalt composite oxide for use as a cathode active material in a lithium ion secondary battery, which has the general formula:
LixMyCo1xe2x88x92yO2 xe2x80x83xe2x80x83(I) 
in which M represents at least one metallic element selected from the group consisting of Al, Ti, Mn, Mo and Sn etc.; x is a numeral in the range of 0.8 to 1.2; and y is a numeral in the range of 0.001 to 0.10.
According to the invention, the substituted lithium cobalt composite oxide is obtained by mixing powders of a lithium compound, a cobalt compound and a compound of the element M in an aliphatic lower alcohol of 1 to 3 carbon atoms as a solvent in such a molar ratio of the elements, Li, Co and M as is coincident to the above-mentioned general formula (I), drying the resulting mixture and firing the dried mixture at a temperature in the range of 600xc2x0 C. to 1100xc2x0 C. under an oxidative atmosphere.
The invention further provides a lithium ion secondary battery which makes use of the substituted lithium cobalt complex oxide as a cathode active material.